Dual stage combustion furnace

ABSTRACT

A dual stage combustion furnace has primary and secondary combustion chambers. The primary combustion chamber contains a solid fuel, such as wood or coal. The secondary combustion chamber is formed adjacent to and in communication with the primary combustion chamber for containing and igniting volatile combustion gases produced in the primary chamber. A plurality of hollow members, which provide a grate, extend through the primary chamber, and into the secondary chamber. Volatile gases given off in the primary combustion chamber are then ignited and burned in the secondary combustion chamber upon combination with heated air passing through the hollow grate members.

This is a continuation of application Ser. No. 634,096, filed 8/30/84,now abandoned, which is a division of application Ser. No. 501,738,filed 6/6/83, now U.S. Pat. No. 4,484,530.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improvements in solid fuel burningstoves and furnaces and, more particularly, to stoves and furnaceswherein gaseous volatiles given off by primary combustion are themselvesignited and burned in a secondary combustion chamber.

2. Description of the Prior Art

Numerous attempts have been made in the past to provide a stove orfurnace which efficiently burns solid fuels, such as coal or wood. Whenburning such solid fuels, coal in particular, some form of support, suchas a grate, must be provided upon which the fuel rests. Further, anoutside air intake must be situated below or above the grate. Air isdrawn in through the intake and passes up the grate and existing spacesin the fuel, thereby providing oxygen necessary to support combustion.The resulting fire in the combustion chamber provides the heat to warmliving or working quarters. However, in previously known solid fuelstoves and furnaces a great deal of the heat potential is lost up thechimney without ever being efficiently converted to heat. In manyinstances, the byproducts caused by initial combustion are given off asgases which rise up the chimney where they cool and form deposits,commonly known when burning wood, as creosote. The deposits on thechimney can be quite dangerous as they may themselves be ignitedresulting in a chimney fire.

Examples of various types of prior furnaces such as described above, maybe found in the following U.S. Pat. Nos. 163,009; 229,684; 707,364;1,484,908; 1,485,545; and 2,070,536.

SUMMARY OF THE INVENTION

The present invention provides a solid fuel burning stove or furnacewhich provides heat from initial combustion and then, in secondarycombustion, ignites and almost completely burns the gases given off bythe initial combustion. Thus, more BTU's per pound of fuel areavailable. Further, the solid fuel stove or furnance constructedaccording to principles of the present invention is simple and selfregulating, needing no damper control, nor sophisticated air intakecontrol. The amount of combustion air is automatically regulated by thetemperature of the combustion taking place. As the fire becomes hotterthe amount of air taken in is decreased and the fire intensity isreduced. However, as the fire intensity decreases, the air intakeincreases which brings the fire intensity back to temperature.

Still further, the furnace described herein features a grate which isself cooling and therefore impervious to the normally deleterious effectof combustion occuring in direct contact thereon.

A further benefit may be found in that the fuel burned has been burnedso completely that the gases given off are less noxious than wouldotherwise be the case. Therefore, with soft coal any deleterious effecton the ecology of coal burning is reduced. A still further benefit isderived from the fact that the furnace described and claimed herein isfar more efficient than the solid fuel furnaces known heretofore in thatthe consumption of fuel will be substantially reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cut away, perspective view of my solid fuelburning furnace;

FIG. 2 is a side view, in section, of the furnace shown in FIG. 1;

FIG. 3 is a sectional view taken along line 3--3 of FIG. 2;

FIG. 4 partial side view in section of another embodiment of my furnace;and

FIGS. 5 and 6 are schematic views of still further embodiments of myfurnace.

DETAILED DESRIPTION OF THE INVENTION

Referring to FIGS. 1-3 of the drawings, one embodiment of my new selfregulating solid fuel furnace may be clearly seen. The furnace 10 is ofa generally cylindrical shape with inner and outer cylinders or drums 11and 13. Other shapes (e.g. rectangular or square) may be used. A primarycombustion chamber 12, has a fuel access door 14 located at its uppermost portion 16. A funnel shaped member 18 is mounted at its largerdiameter 20 to the interior surface 22 of primary combustion chamberwith the smaller opening 24 thereof being located downward from the fuelaccess door with respect to larger opening 20. A plurality of apertures26 are formed in the primary combustion chamber 12 just below where thelarger diameter 20 of funnel 16 is mounted. A plurality of hollowmembers, or tubes, 30 extends through front apertures 32 and rearapertures 34 of the primary combustion chamber. The tubes 30 are locatedin apertures 32 and 34 and can be rotated if desired. The frontapertures 32 are lower in the chamber 12 than the rear apertures 34,which results in the tubes 30 tilting upward toward the rear. The tubes30 provide a grate, upon which the solid fuel is supported.

A secondary combustion chamber 36 is formed about and joined to theprimary combustion chamber 12 and creates an annular air space 38therebetween. The secondary combustion chamber 36 has an exhaust port40. A flue pipe 43 is fitted to the exhaust port 40 for connection toappropriate venting means. Secondary combustion chamber 36 further hasan ash clean out door 42 which permits access into the bottom portion 44of primary combustion chamber 12. The front ends or lower portions ofthe tubes 30 extend through the secondary combustion chamber 36 so as tobe in juxtaposition with ash door 42. The ash door 42 is fitted looselyto chamber 36 such that ambient air may be drawn in around the door 42into hollow members 30. Alternatively, openings 45 in the door may beprovided.

In operation, fuel is introduced to the primary combustion chamber 12through fuel access door 14. The fuel passes through funnel 18 and restson the tubes 30 providing the grate. The fuel initially charged issufficient to form a pile above the hollow grate tubes 30 which reachessmall diameter end 24 of funnel 18, thereby blocking it. A fire is thenstarted in the fuel pile at the lower portion supported by the gratetubes 30. As previously stated, the solid fuel is piled to such anextent that it blocks the funnel. Therefore, no air is taken into theprimary combustiton chamber 12 through, for instance, the fuel accessdoor 14; nor are gases expelled therethrough. Instead, combustion air istaken in through ash door 42.

As the fire intensifies, gasses created by the burning fuel pile arevented through the apertures 26 in primary combustion chamber 12. As thefire burns, a portion of the air drawn in through the ash door 42 isdirected into and through the grate tubes 30. The area 46 between theprimary combustion chamber 12 and the secondary combustion chamber 36has a seal 48 so that no primary combustion air is taken initially intothe secondary chamber 36. The primary combustion air, however, takeninto the hollow grate tubes 30 is heated by the combustion taking placeabove. The incline of the grate members explained earlier, and evidentin FIG. 2, assists in moving the air being heated in the grate tubes 30from their primary air intake ends at 50 to exhaust at their oppositeends 52 into the space 38 of the secondary combustion chamber 36. Theair moving through the tubes 30 further acts to cool them, substantiallyincreasing their lifespan as compared to solid grate bars.

In the process of burning, the fuel pile gives off volatile gases 54which are exhausted through primary combustion chamber apertures 26 intothe space 38 of secondary combustion chamber 36. The air being exhaustedinto the secondary chamber 36 by the hollow grate tubes 30 is superheated as it rises in the region around the primary combustion chamber.This super heated air is of such a temperature that when it encountersthe volatile exhaust gases 54 it causes them to ignite. This ignition ofthe volatile gases produces additional heat from a source which isnormally exhausted as a pollutant into the atmosphere. Additionally, aplurality of heat transfer fins 58 may be formed on the outside 60 ofprimary combustion chamber 12. These fins aid in the secondarycombustion proces by ensuring that the air is sufficiently super heatedto cause secondary combustion. Also, the secondary combustion chambermay have an insulative blanket not shown added to retain heat absorbed.After ignition of the volatile gases 54 has taken place, normal exhausttakes place with the fully burned gases 56 being passed out the fluepipe 43.

The apertures 26 are preferably located approximately one half way upthe furnace. Consequently, the heat produced is distributed evenly overa very wide external area, resulting in a most efficient distribution ofheat. If desired a heat exchanger, e.g. a jacket or helical tube, may besecured to the outside of the drum 11. Water may be circulated throughthe exhanger to provide hot water for heating, washing, and the like.Because the furnace is super heated and burns nearly all volatilesubstances, it produces very few products which cause heat exchangers toclog up with soot. The requirement for periodic cleaning is therebyreduced.

A further embodiment of my furnace is shown in FIG. 4 wherein a furnace110 similar to the one previously described, includes a primarycombustion chamber 112 and a secondary combustion chamber 114. Theprimary chamber 112 has a fuel funnel 116, exhaust apertures 118, and arefractory lining 119. Centered at the bottom of the primary chamber 112is a plenum chamber 120 which is connected via intake port 122 andapertured ash door 124 to the source of ambient air 125. Plenum chamber120 has a plurality of hollow grate members 126 which are fixed to andextend radially upwardly thereform. The grate members 126 pass throughapertures 128 formed in the primary chamber 112 and terminate in space130 of secondary chamber 114. The same combustion process as describedin connection with the furnace illustrated in FIGS. 1-3 occurs in thisembodiment; the only difference being that the combustion air is takenin through intake 122 and distributed by plenum 120 through the radiallyupwardly disposed grate members 126.

Another embodiment of my invention is shown schematically in FIG. 5. Adual stage furnace 210 has a primary chamber 212 and a secondary chamber214. In this embodiment, the secondary chamber 214 is mounted inside theprimary chamber 212 rather than vice versa as in the previouslydescribed embodiments. The primary chamber 212 has a fuel access door216 through which fuel is introduced, and an ash door 218 through whichashes may be extracted. The secondary chamber 214 has a plurality ofintake grate tube members 220 which extend radially therefrom, similarto the spokes on a wagon wheel. The intake members 220 are positioned infront apertures 222 and rear appertures 223, respectively, in primarychamber 212 and communicate with the ambient to receive outside air 224which is super heated and circulated to the secondary chamber 214. Thegrate tube members 220 rise slightly from the front apertures 222 to therear apertures 223 in the wall of the secondary chamber 214.

The secondary chamber 214 terminates at an exhaust hood 228 formed inprimary chamber 212. The hood 228 is connected to a flue pipe 230. Asilicon carbide honeycomb 232 may be provided in the flue pipe 230 toconfine heat to the furnace, as more fully described below.

In operation, fuel is introduced through fuel door 216 and supported onthe grate tubes 220. The fire is started, and combustion air drawn inthrough ash door 218. Volatile exhaust from the fuel is collected at theexhaust hood 228. Air taken in through the grate tubes 220 is superheated by the primary combustion and rises up secondary chamber 214where it continues to become hotter. The super heated air contacts thevolatile gases at the exhaust hood 228 and proceeds to cause the gasesto ignite, thereby creating additional heat within the furnace.

In a still further embodiment, schematically shown in FIG. 6, a dualstage furnace 310 comprises a primary combustion chamber 312 which has afuel access door 314, an ash door 316 and a flue pipe 318. A hollow airintake grate 320 made up of tubes, similar to the tubes 30 of the grateof the furnace shown in FIGS. 1-3, is tilted upwardly in the housing ofthe primary combustion chamber 312; rising from the front ash door 316to the wall 322 at the rear of the housing of the primary combustionchamber 312 where it joins a hollow outside chamber 324. A hollowVenturi shaped tube 326 is joined to and extends from the outside hollowchamber 324 into primary combustion housing 312 at the inlet throat 328of an exhaust pipe 318. Ignition of the fuel source gives off heatedgases which contain, as previously explained, unburned volatiles.Outside side air drawn in through hollow grate tubes 326 is superheatedand flows up hollow chamber 324 and passes into Venturi tube 326. Thevolatile gases meet and mix with super-heated air delivered from theVenturi tube 326 in area 328 resulting in secondary combustion. Ahoneycomb 330 may be placed inside the exhaust pipe 318, as shown. Thesecondary combustion aided by the Venturi tube causes the siliconcarbide honeycomb to glow cherry red which thereby acts, possibly as acatalyst, to further ignite and consume remaining unburned volatiles.The honeycomb converter has hexagonal holes. In a preferred embodimentthe honeycomb is 6 inches long, and there are twenty holes per squareinch, which is quite sturdy.

In the furnaces hereinabove described it will be obvious that the hollowgrates may require some manner of poking to dispose of the accumulatedashes. An appropriate movable poker or an array of poker bars may beemployed. The tubes may alternatively be manually rotated through a gearsystem.

The furnace as illustrated and described herein is merely indicative ofvarious forms of my invention. There are, of course, various otherapplications which can be derived. For example, the exhaust pipe heatedsecondary air intake afterburner using the Venturi 326 can be soldseparately and used in retrofitting existing furnaces. It should,therefore, be understood that my invention is not limited thereto butmay be embodied into other furnaces without departing from my inventionas set forth in the following claims.

I claim:
 1. In a solid fuel burning furnace having a primary combustionchamber, said primary combustion chamber having a wall and a hollowgrate supporting a bed of fuel for superheating secondary air passingthere through, the improvement comprising an afterburner which comprisesan exhaust pipe which extends through the wall of said chamber entirelyabove said bed and defines a secondary combustion chamber entirelywithin said primary combustion chamber, said exhaust pipe having athroat with an open end which projects into said primary combustionchamber above said bed, a tube around said exhaust pipe defining apassage for secondary air toward the throat of said exhaust pipe, aconduit between said hollow grate and said passage for communicatingsuperheated air toward said throat, and ignitor means in said exhaustpipe.
 2. The improvement according to claim 1 wherein said igniter is ahoneycomb of silicon carbide which becomes heated to cherry redtemperature and ignites volatile gases passing through said exhaustpipe.
 3. In a solid fuel burning furnace having a primary combustionchamber, said primary combustion chamber having a wall and a hollowgrate supporting a bed of fuel for superheating secondary air passingtherethrough, the improvement comprising an afterburner which comprisesan exhaust pipe which extends through the wall of said chamber abovesaid bed, said exhaust pipe having a throat with an open end whichprojects into said chamber, a tube around said exhaust pipe defining apassage for secondary air towards the throat of said exhaust pipe, saidtube extending beyond said throat and defining a Venturi adjacent theopen end of said throat, a conduit between said hollow grate and saidpassage for communicating superheated air toward said throat, andignitor means in said exhaust pipe.
 4. The improvement according toclaim 3 wherein means are provided defining a hollow chamber adjacentsaid wall of said combustion chamber, said exhaust pipe extendingthrough said hollow chamber and projecting out of said combustionchamber, the end of said tube opposite to said throat being connected tosaid hollow chamber so as to communicate said passage therewith, saidhollow chamber being connected to said hollow grate and defining saidconduit for superheated air therefrom.
 5. In a solid fuel burningfurnace having a primary combustion chamber, said primary combustionchamber having a wall and a hollow grate supporting a bed of fuel forsuperheating secondary air passing there through, the improvementcomprising an afterburner which comprises and exhaust pipe which extendsthrough the wall of said chamber above said bed, said exhaust pipehaving a throat with an open end which projects into said chamber, atube around said exhaust pipe defining a passage for secondary airtoward the throat of said exhaust pipe, a conduit between said hollowgrate and said passage for communicating superheated air toward saidthroat, said conduit communicating with said passage at a locationspaced along said passage away from said throat so as to provide for theflow of said superheated secondary air in a direction opposite to theflow through said exhaust pipe out of said primary combustion chamber,and ignitor means in said exhaust pipe.
 6. In a solid fuel burningfurnace having a primary combustion chamber, said primary combustionchamber having a wall and a hollow grate supporting a bed of fuel forsuperheating secondary air passing there through, the improvementcomprising an afterburner which comprises an exhaust pipe which extendsthrough the wall of said chamber above said bed and defines therein asecondary combustion chamber entirely within said primary combustionchamber and above said bed, said exhaust pipe having a throat with anopen end which projects into said chamber, a tube around said exhaustpipe defining a passage for secondary air toward the throat of saidexhaust pipe, and a conduit between said hollow grate and said passagefor communicating superheated air toward said throat.
 7. In a solid fuelburning furnace having a primary combustion chamber, said primarycombustion chamber having a wall and a hollow grate supporting a bed offuel for superheating secondary air passing there through, theimprovement comprising an afterburner which comprises an exhaust pipewhich extends through the wall of said chamber above said bed, saidexhaust pipe having a throat with an open end which projects into saidchamber, a tube around said exhaust pipe defining a passage forsecondary air toward the throat of said exhaust pipe, a conduit betweensaid hollow grate and said passage for communicating superheated airtoward said throat, said tube extending beyond said throat defining aVenturi adjacent the open end of said throat.
 8. The improvementaccording to claim 7 wherein means are provided defining a hollowchamber adjacent said wall of said combustion chamber, said exhaust pipeextending through said hollow chamber and projecting out of saidcombustion chamber, the end of said tube opposite to said throat beingconnected to said hollow chamber so as to communicate said passagetherewith, and said hollow chamber being connected to said hollow grateand defining said conduit for superheated air therefrom.
 9. In a solidfuel burning furnace having a primary combustion chamber, said primarycombustion chamber having a wall and a hollow grate supporting a bed offuel for superheating secondary air passing there through, theimprovement comprising an afterburner which comprises an exhaust pipewhich extends through the wall of said chamber above said bed, saidexhaust pipe having a throat with an open end which projects into saidchamber, a tube around said exhaust pipe defining a passage forsecondary air toward the throat of said exhaust pipe, a conduit betweensaid hollow grate and said passage for communicating superheated airtoward said throat, said conduit communicating with said passage at alocation spaced along said passage away from said throat so as toprovide for the flow of said superheated secondary air in a directionopposite to the flow through said exhaust pipe out of said primarycombustion chamber.